In antenna design, the space available is an important factor. A good-quality antenna is relatively easy to make if there are no size restrictions. In radio apparatuses, especially in mobile phones, the antenna is preferably placed within the covering of the device for convenience. As the devices get smaller and smaller, the space for the antenna keeps shrinking, too, which means tighter requirements in antenna design. Another factor contributing to this is that often an antenna has to be capable of operating in two or more frequency bands.
An antenna with satisfactory characteristics which fits inside a small device is in practice most easily implemented as a planar structure: The antenna comprises a radiating plane and a ground plane parallel thereto. In order to make impedance matching easier, the radiating plane and ground plane are usually interconnected at a suitable point by means of a short-circuit conductor, producing a PIFA (planar inverted F antenna) type structure. The size of the ground plane naturally has significance as regards the antenna characteristics. As in the case of a monopole whip, an ideal planar antenna also has a very large ground plane. As the ground plane gets smaller, the resonances of the antenna get weaker and, partly for that reason, the antenna gain decreases. If one keeps on reducing the size of the ground plane, it may at some point function as a radiator, thus changing the antenna characteristics in an uncontrolled manner.
FIG. 1 shows a known PIFA-type internal planar antenna. It includes a circuit board 105 of the radio apparatus, which board has a conductive upper surface. That conductive surface functions as a ground plane 110 for the planar antenna. At the other end of the circuit board there is a radiating plane 120 of the antenna, supported above the ground plane by a dielectric frame 150. The antenna structure further comprises, near a corner of the radiating plane, an antenna feed conductor 131 joining thereto, and a short-circuit conductor 132 connecting the radiating plane to the ground plane at a point S. From the feed conductor there is a via hole, isolated from the ground, to an antenna port on the lower surface of the circuit board 105. In the radiating plane there is a slot 125 which starts from an edge of the plane near the feed conductor 131 and ends up in the inner region of the plane near the opposite edge. The slot 125 divides the radiating plane, viewed from the short-circuit point, into two branches B1, B2 of different lengths. The PIFA thus has two separate resonance frequencies and respective operating bands.
A disadvantage of the antenna of FIG. 1, when the radio apparatus in question is very small, is that it has somewhat modest electrical characteristics. This is caused by the smallness of the ground plane, as described above, and also by the limited height of the antenna, as the radio apparatus is made relatively flat.